As a method for producing a printed wiring board or a lead frame, there is a subtractive method in which an etching resist layer is formed in a circuit portion of an insulating substrate having a conductive layer formed thereon or a conductive substrate, and a conductive layer in an exposed non-circuit portion is removed by etching to form a conductor pattern. Further, there is also an additive method or semi-additive method in which a conductive layer is formed in a circuit portion of an insulating substrate by a plating method.
Meanwhile, with the downsizing and functional enhancement of electronic instruments in recent years, the density of printed wiring boards and lead frames for use inside instruments is increased, and their conductor pattern is decreased in size. At present, conductor patterns having a conductor width of 50 to less than 80 μm and an inter-conductor gap of 50 to 80 μm are produced by a subtractive method. Further, far higher densities and finer wirings have come to be employed, and there are demanded ultrafine conductor patterns having a conductor width or inter-conductor gap of less than 50 μm. Demands are accordingly intensified with regard to the accuracy of a conductor pattern and impedance. For forming such a fine conductor pattern, a semi-additive method has been conventionally studied as a substitute for the subtractive method, while it has a problem that the number of its production steps is greatly increased or a problem that the bonding strength of electrolytically plated copper is insufficient, etc. It has therefore become mainstream to produce printed wiring boards or lead frames by the subtractive method.
In the subtractive method, the etching resist layer is formed by a photofabrication method having the exposure development step using a photosensitive material, a screen printing method, an inkjet method, etc. Of these, a method using a sheet-shaped optically crosslinkable resin layer called a negative type dry film resist in the photofabrication method is suitably used since it is excellent in handling property and enables the protection of through holes by tenting.
In the method using an optically crosslinkable resin, an optically crosslinkable resin layer is formed on a substrate and the steps of exposure and development are carried out to form an etching resist layer. For forming a fine conductor pattern, it is necessary and indispensable to form a fine etching resist layer. For this purpose, the resist film needs to be decreased in thickness such that it is as thin as possible. When a dry film resist that is normally used as an optically crosslinkable resin layer is decreased in thickness such that it has a thickness, for example, of 10 μm or less, the inclusion of air bubbles having foreign particles as cores or the decrease of a concavoconvex form tracking property causes a problem that the resist layer peels or breaks, and it has been difficult to form a fine etching resist layer.
For overcoming the above problem, there has been proposed a method in which a dry film resist having a thickness of 25 μm or more is bonded to a substrate, the thickness of the dry film resist is then decreased to about 10 μm with an alkali aqueous solution, and then the exposure and development for a circuit pattern are carried out to form an etching resist layer (e.g., see JP 2004-214253A).
However, the method in the JP 2004-214253A has some problems. The first problem is that the formation of a thin film sometimes becomes non-uniform or that it is sometimes difficult to carry out the continuous formation of thinner films. As described in JP 2004-214253A, when the thickness of film of a dry film resist is decreased by the use of an alkali aqueous solution containing 1 mass % of sodium carbonate, the solubility speed of the dry film resist varies to a great extent due to differences of liquid flows in the substrate surface, it is hence difficult to form a uniformly thinner film from the dry film resist, and the in-plane width of a conductor formed by etching sometimes varies. Further, since a dissolved dry film resist is dissolved in the alkali aqueous solution, the dissolving capability of the alkali aqueous solution changes when continuous treatment is carried out, so that there is caused a problem that the continuous formation of thinner films can be no longer carried out.
The second problem is that when a printed wiring board having a through hole and non-through hole called a blind via hole (to be referred to as “hole” hereinafter) is made, a conductive layer inside the hole sometimes cannot be protected. In the subtractive method, the conductive layer formed inside the hole is protected by keeping an etching liquid from entering the hole according to a tenting method using a dry film resist. However, a dry film resist of which the thickness is decreased all over the surface as described in JP 2004-214253A has a problem that the conductive layer inside the hole is sometimes etched during the etching since the dry film resist tented over the hole breaks.
For overcoming the second problem, for example, there has been proposed a method for producing a circuit board, in which a dry film resist formed by stacking a first photosensitive resin layer and a second photosensitive resin layer which are different in sensitivity is used, a light quantity necessary for crosslinking the second photosensitive resin layer is applied in the form of a predetermined pattern, and a light quantity necessary for crosslinking both of the first photosensitive resin layer and the second photosensitive resin layer is applied over the hole and its surrounding area, thereby to form an etching resist layer having one thickness on a conductor-pattern-forming region and another thickness on the hole and its surrounding area on the surface of a conductive layer (e.g., see JP 2005-136223). However, the method described in JP 2005-0136223 requires a special dry film resist having two photosensitive resin layers different in sensitivity, and has a problem that it is difficult to adjust the alkali developability, application property, etc.
The third problem is an exposure-related problem. As an exposure method for a circuit pattern, there are known a reflected image exposure method using, as a light source, a xenon lamp, a high-pressure mercury lamp, a low-pressure mercury lamp, an ultra-high pressure mercury lamp or a UV fluorescent lamp, a one-surface or double-surface contact exposure method using an exposure mask, a proximity method, a projection method, a laser scanning exposure method, etc. In the laser scanning exposure method, the beam diameter of laser is large, and it is sometimes difficult to comply with a high resolution. Further, the laser scanning exposure method requires the use of use a special dry film resist having sensitivity to wavelengths inherent to a laser light source, it is not any generally practiced method. In the proximity method and the projection method, a space is there between an exposure mask and a dry film resist, and the resolution is sometimes decreased due to the diffracted light of exposure light, so that they are not any generally practiced methods, either. On the other hand, in the contact exposure method, the space between an exposure mask and a dry film resist is substantially zero, and when a light source of parallel rays is used, it can be said to be an exposure method that is the most suitable for a high resolution.
In JP 2004-214253, however, when an exposure for a circuit pattern is made according to a contact exposure method after a dry film resist is decreased in thickness, light reflected on a substrate surface is scattered (halation), which causes a problem that the resolution of a pattern is deteriorated. The adverse effect by the above halation on the substrate surface is negligibly small when the resist film has a large thickness, while it clearly appears when the resist film is an ultra-thin film having a thickness of 10 μm or less, and after a development step, it is difficult to resolve a narrow space having a size of 20 μm or less, a further improvement being required.
The fourth problem is also concerned with an exposure step. In JP 2004-214253, after a dry film resist is decreased in thickness, the surface of photo-crosslinkable resin layer of the dry film resist is exposed, so that the surface of the photo-crosslinkable resin layer is liable to have a hit mark or scratch due to a foreign matter when it is strongly intimately attached to an exposure mask in vacuum as it is. Further, since the photo-crosslinkable resin layer is in an exposed state, defects by the adherence of a foreign matter or scratches are liable to occur in a carrying step or inputting and receiving steps, etc. Further, when the surface of photo-crosslinkable resin layer of the dry film resist has tacking nature, the surface of the photo-crosslinkable resin layer and the surface of an exposure mask become poor in slipperiness when they are intimately attached to each other in vacuum, and a failure in the intimate attaching in vacuum takes place locally, so that a resist defect by the leak of light is liable to take place. Further, the photo-crosslinkable resin layer may be sometimes transferred to contaminate the exposure mask, or the resist layer may sometimes peel off, so that there is a problem that extra cleaning work is required.